1. Technical Field
This invention relates to the technology of enhancing valve seat performance in automotive engine components and more particularly to techniques for adding materials that define enhanced valve seats in aluminum components, such as in an aluminum cylinder head.
2. Discussion of the Prior Art
Valve seat inserts for aluminum alloy engine heads have been used for some time to reinforce the valve seat areas that are continuously impacted by valves under high temperature and shock. These inserts are usually made of iron, or nickel-based powder-metal compacts to withstand the heat, stress and impact loading that is experienced in such applications. The inserts are welded, pressed into, or shrunk-fit into pre-machined pockets of the head seat support. Although such inserts enhance wear resistance beyond that of the parent aluminum, they may limit engine combustion parameters by restricting heat flow from the valves into the cylinder head and ultimately to the cooling jacket. The increase in temperature can result from two aspects. First, there can be gaps as large as 50-150 micrometers between the insert and parent support metal of the cylinder head; such gaps prevent efficient heat evacuation away from the seat through the head during combustion, consequently increasing the temperature of the valves in contact with such seats. Secondly, inserts need to have a significant thickness to assure adequate rigidity during mechanical installation; such thickness contributes to thermal resistance, thus limiting heat flow from the valves. As a consequence engine operation may have to be modified to prevent extreme temperatures from being experienced by the valves, such as by restricting the degree of spark advance thereby limiting the available horsepower and torque.
Attempts by the prior art to reduce thermal barriers created by powder metal inserts have included cast-in-place inserts, hot pressed/forged inserts, and laser cladding. Cast-in-place inserts and hot press/forged inserts lead to unacceptable tolerance and diffusion of aluminum into the inserts which reduces their strength, wear and corrosion resistance; laser cladding is usually of preplaced or simultaneously fed powders or wires of hard facing alloys into the valve seat region. Laser cladding can reduce the valve operating temperature by as much as 150.degree. F. (see U.S. Pat. Nos. 4,378,332 and 4,723,518). Unfortunately laser cladding has some major disadvantages: (a) it is very expensive and difficult to adapt to high volume manufacturing; (b) its use requires castings of extraordinary quality since any residual gas in the head casting may be liberated during cladding and cause porosity or expulsion of the clad; (c) laser cladding introduces a significant amount of heat into the seat supporting region which significantly modifies metallurgy of the underlying aluminum alloy; and (d) special treatment may be needed for the surface prior to cladding to provide for laser coupling to the aluminum and allow the process to be carried out.